The loop needs to be short because the loopback buffer is only active in loops of 64 or fewer entries (usually fewer real instructions, something like 40 or so). Moreover, Skylake introduced one loopback buffer per thread, instead of the previous loopback buffer shared between both threads.

My guess is that is where the bug is; the behavior for partial register access stalls---insert one extraneous uop to combine, e.g., ah with rax---is unchanged since Sandy Bridge.